The present invention concerns translator devices and more particularly small-scale translator devices which are used for positioning or displacing with a high degree of precision items such as for example components which are to be machined in within a micrometer, components of an item of optical equipment (mirrors, prisms, etc), optical fibers to be aligned for end-to-end assembly thereof, or plates of semiconductor material, commonly referred to as wafers, in the production of integrated circuits.
More precisely the invention relates to actuators or feed arrangements having a micrometric screw, with which such translator devices are fitted.
In the field of medium-precision mechanical engineering and in particular in the field of machine tools, a degree of precision and reproducibility of the order of 5 to 50 micrometers is generally considered to be adequate so that the problems of weight and size are not considerations of prime importance. Therefore, actuators comprising an assembly made up of a screw and a nut forming a carriage have already long been used in that field for translator devices. The screw can rotate in bearings in a frame structure, while being axially immobilized by abutments. The carriage is guided by parallel sliding guides which compel it to move parallel to the axis of the screw. Added thereto are means for driving the screw in rotation in both directions, formed for example by a motor and a train of gears, although the drive means may also be manual.
That simple design of an actuator is appropriate provided that a certain amount of play in regard to the position of the movable components is tolerated.
If the axial play of the screw and the carriage is to be eliminated, it is possible in addition to provide a support spring around the screw and between the carriage and the frame structure, but that design configuration is far from perfect as the support force of the spring then depends on the position of the carriage. It is possible to improve the level of precision by separately taking up the axial play of the screw and the axial play of the carriage with respect to the screw by means of a second spring which bears against a follower nut also mounted on the screw, which obviously complicates the design.
Moreover, such measures are not sufficient to provide a high-performance actuator which must make it possible to achieve a level of precision of the order of a micrometer and even less and a degree of resolution which is ten times better. In that case, all the plays involved must be completely eliminated. A known way of achieving that consists of using prestressed ball-type bearings and slide members for guiding the carriage and possibly also ball-type transmissions between the carriage and the screw.
Now, the presence of such ball-type components is incompatible with the miniaturization which is desirable for uses such as those referred to hereinbefore.
Moreover, the smaller those components are, the more expensive they are, delicate and subject to rapid wear due to the severely hyperstatic character of their connections and the high levels of stress which result therefrom. Consequently the plays which they are required to eliminate re-appear fairly quickly in the course of use of the arrangement and, the more such plays increase, the greater the reduction in the level of precision of the actuator and the translator device of which they are part.
Moreover, when they are motorized, actuators of that kind are provided with electronic end-of-travel safety devices such as switches or photoelectric cells which make it possible to stop the motor before the carriage touches the frame structure but which run the risk of not always operating. When that actually occurs, the carriage comes to bear against the frame structure and the screw which continues to rotate resiliently loads the frame structure until the frictional torques induced by its axial thrust force are sufficient to stop the motor. As the screw is irreversible, the friction persists after the motor has stopped and in general the motor does not have a sufficient level of torque to re-start in the opposite direction, so that the mechanism remains jammed.
When that failure occurs in screw-type actuators of relatively large size, it is easy to unjam the arrangement by manual means, and it is even possible to try to eliminate the risk of failure by supplementing the electronic safety devices with mechanical safety devices.
In contrast, when dealing with actuators of very small dimensions, a manual unjamming operation is very delicate one and the presence of additional mechanical safety devices does not go hand in hand with miniaturization.